Use CDN jQuery | WP

functions.php

<?php
//----------------------------------------//
// Making jQuery load from Google Library //
//----------------------------------------//

add_filter( 'init', 'replace_default_jquery_with_fallback');
function replace_default_jquery_with_fallback() {

	if (is_admin()) {
		return;
	}

    $ver = '1.12.4';
    $migrateVer = '1.4.1';

    // Dequeue first then deregister
    wp_dequeue_script( 'jquery' );
    wp_dequeue_script( 'jquery-migrate' );

    wp_deregister_script( 'jquery' );
    wp_deregister_script( 'jquery-migrate' );

    // Set last parameter to 'true' if you want to load it in footer
    wp_register_script( 'jquery-core', "//ajax.googleapis.com/ajax/libs/jquery/$ver/jquery.min.js", '', $ver, false );
    // wp_register_script( 'jquery-mask', "//cdnjs.cloudflare.com/ajax/libs/jquery.mask/1.14.10/jquery.mask.js", '', '1.14.10', false );
    wp_register_script( 'jquery-migrate', "//cdnjs.cloudflare.com/ajax/libs/jquery-migrate/$migrateVer/jquery-migrate.min.js", '', $migrateVer, false );

    // Fallback
    wp_add_inline_script( 'jquery-core', 'window.jQuery||document.write(\'<script src="'.includes_url( '/js/jquery/jquery.js' ).'"><\/script>\')' );
    wp_add_inline_script( 'jquery-migrate', 'window.jQuery||document.write(\'<script src="'.includes_url( '/js/jquery/jquery-migrate.min.js' ).'"><\/script>\')' );

    wp_enqueue_script ( 'jquery-core' );
    // wp_enqueue_script ( 'jquery-mask' );
    wp_enqueue_script ( 'jquery-migrate' );
}

Updated for WP 5.7

If the above doesn't work, use:

//----------------------------------------//
// Making jQuery load from Google Library //
//----------------------------------------//

add_filter( 'wp_enqueue_scripts', 'replace_default_jquery_with_fallback');
function replace_default_jquery_with_fallback() {
    $ver = '1.12.4';
    $migrateVer = '1.4.1';
    // Dequeue first then deregister
    wp_dequeue_script( 'jquery' );
    wp_dequeue_script( 'jquery-migrate' );
    wp_deregister_script( 'jquery' );
    wp_deregister_script( 'jquery-migrate' );
    // Set last parameter to 'true' if you want to load it in footer
    wp_register_script( 'jquery', "//ajax.googleapis.com/ajax/libs/jquery/$ver/jquery.min.js", '', $ver, false );
    wp_register_script( 'jquery-migrate', "//cdnjs.cloudflare.com/ajax/libs/jquery-migrate/$migrateVer/jquery-migrate.min.js", '', $migrateVer, false );
    // Fallback
    wp_add_inline_script( 'jquery', 'window.jQuery||document.write(\'<script src="'.includes_url( '/js/jquery/jquery.js' ).'"><\/script>\')' );
    wp_add_inline_script( 'jquery', 'window.jQuery||document.write(\'<script src="'.includes_url( '/js/jquery/jquery-migrate.min.js' ).'"><\/script>\')' );
    wp_enqueue_script ( 'jquery' );
    wp_enqueue_script ( 'jquery-migrate' );
}